Robotics is a growing, and increasingly important, field in industrial, medical, scientific, and other applications. In many cases, in which a robot arm or a tool attached thereto contacts a workpiece, the force and/or torque applied must be closely monitored. Accordingly, a force/torque sensor is an important part of many robotic systems.
One conventional type of force/torque sensor uses strain gages to measure the deformation of small beams connecting two mechanical parts—one connected to the robot arm and the other connected to a robotic tool (or a mechanical coupling to the tool). For example, a central “hub,” referred to in the art as a Tool Adapter Plate (TAP) is connected to a tool. Another body arranged annularly around, and spaced apart from, the TAP, referred to in the art as a Mounting Adapter Plate (MAP), is connected to a robotic arm. The MAP and TAP are connected to each other by a plurality of relatively thin (and hence mechanically deformable) beams, arranged radially around the TAP—in some cases resembling spokes of a wheel. Relative force or torque between objects respectively attached to the TAP and MAP attempt to move the MAP relative to the TAP, resulting in slight deformation, or bending, of at least some of the beams.
Strain gages are typically affixed to all four surfaces of each beam, nominally in the center of each respective surface. The gages translate tensile and compressive strains at the beams' surfaces, caused by mechanical deformation of the beams, into electrical signals. Once calibrated, signals from all four strain gages on all beams are processed together to resolve the magnitude and direction of relative force and/or torque between the robot arm and tool (and hence the force/torque applied through the tool to a workpiece).
Safety is a paramount concern in any industrial environment in which both humans and robots operate. A force/torque sensor on a robotic is an important aspect of maintaining safety, as it allows the robotic control system to detect a collision or contact with an object (which may be a person), and interrupt ongoing movement to avoid possible damage. It is thus critical that the force/torque sensor be able to accurately measure and report applied loads at all times. If a force/torque sensor were to experience a fault—such as, for example, catastrophic failure of a strain gage (open circuit), partial strain gage failure (resistance offset), strain gage delamination, yield of metal components such as a deformable beam, wiring faults (open or short circuit), or failure of a measurement/processing circuit component—the operational effect would be the same as having a robot without a force/torque sensor at all; this is an unacceptable safety risk. Accordingly, a force/torque sensor, for at least some applications, must be capable of detecting and reporting any faults, even if the damage occurs when the system is not powered on.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.